EP1992958B1 - Method and system for satellite positioning - Google Patents

Description

The present invention relates to terrestrial satellite positioning systems.

Satellite positioning systems are known to have a constellation of navigation satellites placed in mid-altitude orbits (of the order of 25,000 km) around the Earth. These navigation satellites and their orbits are generally referred to in the art as "MEO satellites" and "MEO orbits", respectively ( M edium E arth O rbit). MEO satellites are distributed uniformly in several orbital planes, so that, at any point on the Earth, a user can see several MEO satellites, that is to say, be in direct lines with them (at least three , but four if the user wants to know his altitude) and deduce his own terrestrial coordinates.

Each MEO satellite carries an atomic clock of high stability and accuracy, as well as electronic equipment that sends terrestrial users a message with the corrected time (reduced to a common reference) and the ephemeris of the satellite. From these messages received from several MEO satellites, the user determines its distance to the various satellites in visibility and therefore deduces its position in terrestrial coordinates. To ensure the nominal operation of the MEO satellite constellation, a ground control center performs orbit and time measurement on board for each MEO satellite. This control center calculates the ephemeris of the MEO satellites and the correction of the time to be broadcast by each MEO satellite for the hours to come.

The main disadvantage of these terrestrial positioning systems is that the quality of the positioning of the terrestrial users is based on the quality of the ephemeris and the corrected time diffused by each MOE satellite. Also, as a result of a control center error or a failure onboard an MEO satellite, it may occur that the actual position, the corrected time and / or ephemeris broadcast by one or more satellites MEO become untrue, resulting in a positioning error for all users in the satellite's visibility area. The error will eventually be detected and corrected by the control center that monitors the MEO satellites, but in the meantime some users who use the system for important navigation functions, for example, may find themselves in a difficult position. serious insecurity.

Several solutions have already been proposed to change the design of the MEO satellites and enslave them on external time references in order to increase the reliability of the messages transmitted by the MEO satellites. However, in all these solutions, the detection of anomalies on the MEO satellites is provided by a network of monitoring stations on the ground, which transmit to the control center information on these anomalies, and the correction of these is entrusted to said center control. In addition, to be effective, these solutions implement at least fifty monitoring stations permanently connected to said control center by multiple channels in parallel. This results in high installation and operating costs.

In addition, the detection of anomalies by a network of terrestrial monitoring stations results in a high dilution of accuracy of the restitution of the position of the MEO satellites. Indeed, for a particular MEO satellite, all distance measurements are made from ground stations which are all located in a reduced solid angle because the Earth's diameter is small relative to the diameter of the MEO orbits. The accuracy in the horizontal plane is therefore limited, reducing the reliability of the instantaneous detection of anomalies.

Furthermore, all measurements made by such a network of terrestrial monitoring stations are disturbed by local effects (tropospheric, ionospheric, multi-path errors), which causes measurement errors that degrade the reliability of the anomaly detection and risk generate false alarms, if a tight tolerance threshold has been set for these errors.

An additional disadvantage of anomalous detection by a network of terrestrial monitoring stations is the latency between the moment an anomaly occurs on an MEO satellite and the moment when the user is finally informed of this anomaly. In fact, the detection / notification chain includes the global network of stations, the control center, which from all the measurements of the stations will detect the anomaly, the broadcasting stations to the satellites of the warning message and the system repetition of the alert message aboard the MEO satellite. This long and complex chain induces latency between the occurrence of the anomaly and the notification to users that are difficult to comply with the highest operational safety requirements, such as those applicable in the aeronautical field.

The present invention aims to overcome these disadvantages.

• on répartit dans l'espace une pluralité de balises de référence gérées par ledit centre de contrôle terrestre et aptes à émettre chacune des secondes informations d'heure et de position ainsi qu'un signal radioélectrique spécifiques ; et
• à bord de chaque satellite de navigation :
  • ■ on détecte lesdites secondes informations d'heure et de position émises par au moins certaines desdites balises de référence ;
  • ■ à partir desdites premières et secondes informations d'heure et de position, on calcule des premières valeurs et des deuxièmes valeurs respectivement représentatives des distances entre ledit satellite de navigation et chacune desdites balises et de la vitesse de variation de ces distances ;
  • ■ on mesure la variation de fréquence doppler apparaissant dans lesdits signaux radioélectriques émis par lesdites balises de référence ;
  • ■ à partir desdites variations de fréquence doppler, on calcule des troisièmes valeurs et des quatrièmes valeurs respectivement représentatives des distances entre ledit satellite de navigation et chacune desdites balises et de la vitesse de variation de ces distances ;
  • ■ pour chaque balise de référence, on compare ladite première valeur de distance et ladite troisième valeur de distance, ainsi que ladite deuxième valeur de vitesse de variation de distance et ladite quatrième valeur de vitesse de variation de distance ;
  • ■ on engendre un signal représentatif des résultats desdites comparaisons ; et
  • ■ ledit satellite de navigation émet ce signal de résultat de comparaison en direction dudit utilisateur, au moins dans le cas où ladite comparaison révèle une anomalie.

To this end, according to the invention, the method for positioning a user on the Earth, said method implementing a plurality of navigation satellites which are placed in medium altitude orbits and which are managed by at least one terrestrial control center, each of said navigation satellites emitting first time and position information of its own, is remarkable in that:

• a plurality of reference beacons managed by said terrestrial control center and able to transmit each of the seconds are distributed in space specific time and position information and a radio signal; and
• on board each navigation satellite:
  • Detecting said second time and position information transmitted by at least some of said reference beacons;
  • From said first and second time and position information, first values and second values respectively representative of the distances between said navigation satellite and each of said beacons and the speed of variation of these distances are calculated;
  • The doppler frequency variation occurring in said radio signals emitted by said reference beacons is measured;
  • From said Doppler frequency variations, third values and fourth values respectively representative of the distances between said navigation satellite and each of said beacons and the speed of variation of these distances are calculated;
  • For each reference beacon, comparing said first distance value and said third distance value, as well as said second distance change rate value and said fourth distance change rate value;
  • A signal representative of the results of said comparisons is generated; and
  • Said navigation satellite transmits this comparison result signal towards said user, at least in the case where said comparison reveals an anomaly.

Thus, thanks to the present invention, each MEO navigation satellite is in unidirectional communication with said transmitting beacons and can autonomously verify that the essential information that it transmits to the users is valid at every moment. If the verification is interrupted or if it reveals deviations from this essential information, the navigation satellite concerned includes in the latter a message indicating that the said information is either suspect (in which case the verification chain is interrupted or the detected difference is significant but acceptable), either false. Users can immediately either ignore the information from the corresponding navigation satellite, or reduce the weight assigned to it in the position calculation.

Said reference beacons may, at least in part, be arranged on the Earth or, preferably, on board satellites in high orbits (for example of the order of 40,000 km). In the latter case, it is advantageous for said satellites carrying beacons to be geosynchronous so as to remain constantly in view of said control center. In the case where the high orbits are not geosynchronous, several stations are provided around the earth and connected to said control center.

It will be noted that, by the document US-6,603,426 a method for verifying the integrity of the transmissions of a GPS system is already known. A reference beacon located at a fixed point on the earth in a known position transmits a coded RF signal which is received by GPS satellites. The signal allows each satellite to calculate the distance separating it from the reference beacon. Distance data is transmitted to other GPS satellites via links. When a GPS satellite determines its position relative to the reference beacon, the satellite can calculate its own coordinates in space. This calculated position can be compared to other satellite position data (eg, based on Ephemeris data) to verify the integrity of the GPS system. If there is a deviation greater than a certain predetermined error, the GPS satellite may have a malfunction (integrity). In this In this case, an integrity message may be embedded in the GPS navigation message or in a separate channel.

• ledit système comporte une pluralité de balises de référence réparties dans l'espace et gérées par ledit centre de contrôle terrestre, chaque balise étant apte à émettre des secondes informations d'heure et de position, ainsi qu'un signal radioélectrique ; et
• chaque satellite de navigation comporte de plus :
  • ■ des moyens de réception desdites secondes informations émises par au moins certaines desdites balises de référence, lesdits moyens de réception mesurant la variation de fréquence doppler apparaissant dans lesdits signaux radioélectriques émis par lesdites balises de référence ;
  • ■ des moyens de décodage desdites secondes informations reçues par lesdits moyens de réception ;
  • ■ des moyens de calcul recevant lesdites secondes informations décodées par lesdits moyens de décodage, lesdites variations de fréquence doppler et lesdites premières informations émises par ledit générateur, lesdits moyens de calcul :
    • * calculant lesdites premières, deuxièmes, troisièmes et quatrièmes valeurs à partir desdites premières et secondes informations, d'une part, et desdites variations de fréquence doppler, d'autre part ;
    • * comparant, pour chaque balise de référence, ladite première valeur de distance et ladite troisième valeur de distance, ainsi que ladite deuxième valeur de vitesse de variation de distance et ladite quatrième valeur de vitesse de variation de distance ; et
    • * adressant les résultats des comparaisons audit générateur pour que celui-ci les incorpore éventuellement auxdites premières informations.

According to the present invention, an MEO satellite positioning system comprising a plurality of navigation satellites which are placed in medium altitude orbits and which are managed by at least one terrestrial control center, each of said navigation satellites having a clock atomic, a remote control receiver-decoder issued by said terrestrial control center, a generator of first time and position information connected to said clock and said receiver-decoder, and a transmitter transmitting said first information to said user, is remarkable in that:

• said system comprises a plurality of reference beacons distributed in space and managed by said terrestrial control center, each beacon being able to transmit second time and position information, as well as a radio signal; and
• each navigation satellite also includes:
  • Means for receiving said second information transmitted by at least some of said reference beacons, said receiving means measuring the Doppler frequency variation appearing in said radio signals emitted by said reference beacons;
  • Means for decoding said second information received by said reception means;
  • Calculating means receiving said second information decoded by said decoding means, said Doppler frequency variations and said first information transmitted by said generator, said calculating means:
    • calculating said first, second, third and fourth values from said first and second information, on the one hand, and said Doppler frequency variations, on the other hand;
    • * comparing, for each reference beacon, said first distance value and said third distance value, as well as said second distance change rate value and said fourth distance change rate value; and
    • * addressing the results of the comparisons to said generator so that it eventually incorporates them to said first information.

• si tout est cohérent dans des limites prescrites prédéterminées, alors les moyens de calcul signalent ce fait au générateur de trame de navigation du satellite MEO qui incorpore dans les informations de navigation envoyées vers les utilisateurs un message signalant que les données de positionnement du satellite MEO sont bonnes,
• si la liaison avec les balises est mauvaise (pas assez de balises disponibles ou mauvais bilan de liaison), alors il y a doute sur les données et les moyens de calcul signalent ce fait au générateur de trame de navigation (les premières informations) qui incorpore dans les informations de navigation envoyées vers les utilisateurs un message signalant que la qualité des données de positionnement du satellite MEO est non validée et donc douteuse,
• enfin, si les signaux doppler reçus des balises et/ou la distance du satellite MEO à ces balises sont incohérents avec les données de navigation du satellite MEO, alors le calculateur signale ce fait au générateur de trame de navigation (les premières informations) qui incorpore dans les informations de navigation envoyées vers les utilisateurs un message d'alerte signalant que les données de positionnement du satellite MEO sont fausses.

The decision logic applied by the calculation means for determining the quality and safety of the navigation data may be as follows:

• if everything is coherent within predetermined prescribed limits, then the calculation means signal this fact to the navigation frame generator of the MEO satellite which incorporates in the navigation information sent to the users a message stating that the positioning data of the MEO satellite are good,
• if the connection with the beacons is bad (not enough available beacons or bad link budget), then there is doubt about the data and the means of calculation signal this fact to the navigation frame generator (the first information) which incorporates in the navigation information sent to the users a message indicating that the quality of the positioning data of the MEO satellite is not validated and therefore doubtful,
• finally, if the Doppler signals received from the beacons and / or the distance from the MEO satellite to these beacons are inconsistent with the navigation data of the MEO satellite, then the computer signals this fact to the navigation frame generator (the first information) which incorporates in the navigation information sent to the users an alert message indicating that the positioning data of the MEO satellite is false.

Said first information addressed to said calculation means can be taken directly at the output of said generator of the first information. However, in order to ensure that the quality of the finally transmitted signal is good, it can further be provided in each navigation satellite an auxiliary receiver-decoder for receiving and decoding said first information transmitted by said transmitter, said receiver-decoder auxiliary addressing said first information to said computing means.

The present invention further comprises a navigation satellite as described above.

The figures of the appended drawing will make it clear how the invention can be realized. In these figures, identical references designate similar elements.

The figure 1 is a schematic and partial view of the satellite positioning system according to the present invention.

The figure 2 is the block diagram of the electronic equipment of a navigation satellite according to the present invention.

• des satellites de navigation 1, dits satellites MEO, décrivant des orbites de moyenne altitude 2 (orbites MEO) autour de la Terre T et équipés de systèmes d'antennes 3 ; et
• un centre de contrôle terrestre 4, équipé d'un système d'antennes 5.

The satellite positioning system shown schematically and partially on the figure 1 , includes:

• navigation satellites 1, called MEO satellites, describing mid-altitude orbits 2 (MEO orbits) around Earth T and equipped with antenna systems 3; and
• a terrestrial control center 4, equipped with an antenna system 5.

In known manner, on board each MEO 1 satellite is embedded electronic equipment 6 (see FIG. figure 2 ) adapted to receive, by the appropriate part 3.1 of the antenna system 3, remote controls transmitted by the control center 4, and to emit, by the appropriate part 3.2 of the antenna system 3, positioning signals in the direction of users also on Earth T. Also known, said electronic equipment 6 includes a receiver-decoder remotes 7 connected to the antenna portion 3.1, an atomic clock 8, a generator of navigation frames 9 receiving the signals of the receiver-decoder 7 and the clock 8 and a radio transmitter 10 receiving the navigation frames (corrected time and ephemeris) of the generator 9 and addressing them to the users by the antenna part 3.2.

In accordance with the present invention, the satellite positioning system of the figure 1 further comprises reference beacons 11, equipped with antennas 12 for transmitting a radio carrier with a message comprising their position (in three-dimensional coordinates or in the form of ephemeris) and a time reference signal, repeated at a higher recurrence frequency to the transmission cycle of the signals emitted by the satellites 1 towards the Earth T.

The reference beacons 11 are disposed either on the Earth T or on satellites 14 describing high orbits 15. In the latter case, the antennas 12 of the reference beacons 11 constitute part of the satellite antenna systems 16 .

The signals emitted by the reference beacons 11, via the antennas 12, are managed and synchronized by the terrestrial control center 4.

According to the invention, the signals coming from the reference beacons 11 are used by the satellites 1 to validate the signals that they themselves transmit to the terrestrial users.

To this end, as illustrated schematically on the figure 2 the electronic equipment 6 mounted on board each satellite 1 is complemented by complementary electronic equipment 17 receiving the signals from the beacons 11 via an appropriate part 3.3 of the antenna system 3 of said satellite 1.

The electronic equipment 17 comprises receivers 18 receiving said signals from the beacons 11 via said antenna part 3.3. The receivers 18 measure the Doppler signals of the radio frequencies transmitted by the various reference beacons 11 and send these Doppler signals to a coherence calculator 19 via a link 20. In addition, the receivers 18 send to a decoder 21 the data received from the beacons. reference 11. Said decoder 21 extracts the position and time information received from said reference beacons 11 and the addresses to said coherence calculator 19.

• calcule, pour le satellite 1 qui le porte, les distances aux diverses balises de référence 11, ainsi que la vitesse de variation de ces distances, en prenant en compte, d'une part, les informations décodées de position et d'heure de ces dernières délivrées par le décodeur 21 et, d'autre part, l'heure corrigée à bord et les éphémérides provenant dudit générateur de trames de navigation 9 ;
• vérifie que ces distances et vitesses de variation de distances calculées sont cohérentes avec les signaux doppler mesurés par les récepteurs 18;et
• transmet le résultat de cette vérification de cohérence au générateur de trames de navigation 9 par la liaison 23.

Moreover, by a link 22, the coherence calculator 19 receives the navigation frames generated by the generator 9 of the electronic equipment 6. Thus, the coherence calculator 19:

• calculates, for the satellite 1 which carries it, the distances to the various reference beacons 11, as well as the speed of variation of these distances, taking into account, on the one hand, the decoded information of position and time of the latter delivered by the decoder 21 and, on the other hand, the corrected time on board and the ephemeris coming from said generator of navigation frames 9;
• verifies that these distances and rates of variation of calculated distances are consistent with the Doppler signals measured by the receivers 18;
• transmits the result of this consistency check to the navigation frame generator 9 via link 23.

On the figure 2 , there is further represented a navigation frame receiver-decoder 24, provided with a receiving antenna 3.4, forming part of the antenna system 3. The receiver-decoder 24 is able to capture the navigation frames transmitted by the receiver. radio transmitter 10, through its antenna 3.2, and transmit them to the coherence calculator 19. The receiver-decoder 24 is therefore able to replace and / or complete the link 22.

Claims (10)

1. Method for positioning a user on the Earth (T), the said method implementing a plurality of navigation satellites (1) which are placed in medium-altitude orbits (2) and which are managed by at least one terrestrial control centre (4), each of the said navigation satellites (1) transmitting first time and position information which is individual to it,
   characterized in that:

   - a plurality of reference beacons (11) managed by the said terrestrial control centre (4) and each able to transmit specific second time and position information as well as a specific radioelectric signal is distributed in space; and

   - aboard each navigation satellite (1):

   ■ the said second time and position information transmitted is detected by at least some of the said reference beacons (11);

   ■ first values and second values respectively representative of the distances between the said navigation satellite (1) and each of the said beacons (11) and of the rate of variation of these distances are computed on the basis of the said first and second time and position information;

   ■ the variation in Doppler frequency appearing in the said radioelectric signals transmitted is measured by the said reference beacons (11);

   ■ third values and fourth values respectively representative of the distances between the said navigation satellite (1) and each of the said beacons (11) and of the rate of variation of these distances are computed on the basis of the said variations in Doppler frequency;

   ■ for each reference beacon (11), the said first distance value and the said third distance value, as well as the said second value of rate of variation of distance and the said fourth value of rate of variation of distance are compared;

   ■ a signal representative of the results of the said comparisons is generated; and

   ■ the said navigation satellite (1) transmits this comparison result signal to the said user, at least in the case where the said comparison reveals an anomaly.
2. Method of Claim 1,
   characterized in that at least certain reference beacons (11) are disposed on the Earth (T).
3. Method according to Claim 1,
   characterized in that at least certain reference beacons (11) are disposed aboard satellites (14) in high orbits (15).
4. Method of Claim 3,
   characterized in that the satellites (14) in high orbits (15) are geosynchronous.
5. System for the implementation of the method of positioning a user on the Earth (T) in accordance with any one of Claims 1 to 4, comprising a plurality of navigation satellites (1) which are placed in medium-altitude orbits (2) and which are managed by at least one terrestrial control centre (4), each of the said navigation satellites (1) comprising an atomic clock (8), a receiver-decoder (7) of telecommands transmitted by the said terrestrial control centre (4), a generator (9) of first time and position information which is linked to the said clock (8) and to the said receiver-decoder (7), and a transmitter (10) transmitting the said first information to the said user,
   characterized in that:

   - the said system comprises a plurality of reference beacons (11) distributed in space and managed by the said terrestrial control centre (4), each beacon (11) being able to transmit second time and position information, as well as a radioelectric signal; and

   - each navigation satellite (1) comprises moreover:

   ■ means (18) for receiving the said second information transmitted by at least some of the said reference beacons (11), the said reception means (18) measuring the variation in Doppler frequency appearing in the said radioelectric signals transmitted by the said reference beacons (11);

   ■ means (21) for decoding the said second information received by the said reception means (18);

   ■ computation means (19) receiving the said second information decoded by the said decoding means (21), the said variations in Doppler frequency and the said first information transmitted by the said generator (9) of the first information, the said computation means (19):

   * computing the said first, second, third and fourth values on the basis of the said first and second information, on the one hand, and of the said variations in Doppler frequency, on the other hand;

   * comparing, for each reference beacon (11), the said first distance value and the said third distance value, as well as the said second value of rate of variation of distance and the said fourth value of rate of variation of distance; and

   * addressing the results of the comparisons to the said generator so that the latter possibly incorporates them into the said first information.
6. System according to Claim 5,
   characterized in that the said first information addressed to the said computation means (19) is sampled at the output of the said generator (9) of the first information.
7. System according to Claim 5,
   characterized in that each navigation satellite (1) moreover comprises an auxiliary receiver-decoder (24) for receiving and decoding the said first information transmitted by the said transmitter (10), the said auxiliary receiver-decoder (24) addressing the said first information to the said computation means (19).
8. Navigation satellite for the positioning system of Claim 5, the said navigation satellite (1) comprising an atomic clock (8), a receiver-decoder (7) of telecommands transmitted by a terrestrial control centre (4), a generator (9) of first time and position information which is linked to the said clock (8) and to the said receiver-decoder (7), and a transmitter (10) transmitting the said first information to the said user, and the said navigation system comprising a plurality of reference beacons (11) distributed in space and managed by the said terrestrial control centre (4), each beacon (11) being able to transmit second time and position information, as well as a radioelectric signal,
   characterized in that it moreover comprises:

   - means (18) for receiving the said second information transmitted by at least some of the said reference beacons (11), the said reception means (18) measuring the variation in Doppler frequency appearing in the said radioelectric signals transmitted by the said reference beacons (11);

   - means (21) for decoding the said second information received by the said reception means (18);

   - computation means (19) receiving the said second information decoded by the said decoding means (21), the said variations in Doppler frequency and the said first information transmitted by the said generator (9) of the first information, the said computation means (19):

   ■ computing the said first, second, third and fourth values on the basis of the said first and second information, on the one hand, and of the said variations in Doppler frequency, on the other hand;

   ■ comparing, for each reference beacon (11), the said first distance value and the said third distance value, as well as the said second value of rate of variation of distance and the said fourth value of rate of variation of distance; and

   ■ addressing the results of the comparisons to the said generator so that the latter possibly incorporates them into said first information.
9. Satellite according to Claim 8,
   characterized in that the said first information addressed to the said computation means (19) is sampled at the output of the said generator (9) of the first information.
10. Satellite according to Claim 8,
    characterized in that it moreover comprises an auxiliary receiver-decoder (24) for receiving and decoding the said first information transmitted by the said transmitter (10), the said auxiliary receiver-decoder (24) addressing the said first information to the said computation means (19).

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